1. Field of the Invention
This invention relates to the field of electronically tunable optical filters utilizing acousto-optic (AO) diffraction.
2. Description of Prior Art
Acousto-optic tunable filters (AOTFs) have been constructed so that an incident light of a first polarization is diffracted by an acoustic wave in a birefringent crystal to shift from the first to a second polarization of the light beam for a selected bandpass of optical frequencies. The center wavelength of the passband of this type of filter is electronically tunable by changing the frequency of the acoustic wave within the crystal.
The AOTFs can be divided into two broad categories: collinear and noncollinear. In the collinear filter, the incident and diffracted optical waves inside the birefringent crystal are collinear with the acoustic wave. The diffracted light beam at the selected passband is separated from the incident light beam by crossed polarizers. One significant feature of the collinear AOTF is that the narrow filter bandpass can be maintained for incident light within a large angular aperture. The collinear type of acousto-optic filter is disclosed in an article entitled "Acousto-Optic Tunable Filters," appearing on pages 744-747 in the June, 1969 issue of The Journal of the Optical Society of America (Vol. 59, No. 6), and in U.S. Pat. No. 3,679,288, entitled "Tunable Acousto-Optic Method and Apparatus."
In the noncollinear filter, the optical waves inside the birefringent crystal are noncollinear with the acoustic wave. The diffracted beam at the passband is selected from the incident light beam either by crossed polarizers or by spatial separation. One type of noncollinear AOTF was described in a paper entitled "Noncollinear Acousto-Optic Filter," presented at the 1973 IEEE/OSA conference on laser engineering and applications and in U.S. Pat. No. 3,953,107; entitled "Acousto-Optic Filter." This type of noncollinear AOTF has a small angular aperture and must be restricted to a well-collimated light source. A second type of noncollinear AOTF is disclosed in an article entitled "Noncollinear Acousto-Optic Filter with Large Angular Aperture," appearing on pages 370-372 of the Oct. 15, 1974 issue of the Applied Physics Letters (Vol. 25), and in U.S. Pat. No. 4,052,121 entitled "Noncollinear Tunable Acousto-Optic Filter." In contrast to the first type, this type of noncollinear AOTF has the important feature of large angular aperture. The large angular aperture is due to the proper choices of interaction geometry wherein the tangents to the loci of the incident and diffracted light wave vectors are "parallel," a condition known as non-critical phase matching (NPM).
All of the AOTFs are based on bireflingent diffraction that couples incident light to diffracted light with orthogonal polarizations and different refractive indices. This type of diffraction can occur only in an optical birefringent crystal.
A more comnon type of AO diffraction is the isotropic diffraction that occurs in isotropic media (e.g. crystals or glasses) or in birefringent crystals between incident and diffracted light with the same polarization. In an isotropic diffraction the refractive indices of the incident light and diffracted light are approximately equal. Up to now, isotropic diffraction has not been utilized for AOTF applications.
A different class of AO device is the AO deflector for laser beam scanning. By varying the acoustic frequency the AO deflector scans an incident laser beam into a wide range of resolvable angular positions or spots. A basic performance parameter of the AO deflector is the bandwidth, which is approximately equal to the number of resolvable spots per unit time. The realization of large bandwidth has been one of the major goals in the design of the AO deflector.
One technique of increasing the bandwidth of AO deflector is the use of acoustic beam steering with a phased array of transducers. The simplest phased array employs fixed phase difference of 180 degrees between alternate transducer elements in a planar configuration. By selecting the inter element spacing s to be approximately equal to the characteristic length L.sub.0, where EQU L.sub.0 =n.LAMBDA..sub.0.sup.2 /.lambda..sub.0 ( 1)
where n is the refractive index of the AO medium, .LAMBDA..sub.0 is the acoustic wavelength at the center frequency and .lambda..sub.0 is the optical wavelength, the acoustic beam will be scanned to maintain the phase matching condition over a larger frequency range, thereby increasing the bandwidth of the AO deflector. This technique of increased bandwidth is referred to as tangential phase matching (TPM) since the steered acoustic wavevector is tangential to the locus of the diffracted light vector. A more efficient use of the acoustic power has been demonstrated using a stepped phased array where the height of each step in the phased array is equal to .LAMBDA..sub.1 /2. The phased array is blazed so that the beam steering angle from the transducer plane is zero at the reference acoustic wavelength .LAMBDA..sub.1. Wideband AO deflectors using planar and stepped phased arrays were described in an article by Korpel et al entitled, "A Television Display Using Acoustic Deflection and Modulation of Coherent Light" appearing on pages 1667-1675 in the October 1967 issue of Applied Optics, and another article by E. I. Gordon entitled, "A Review of Acousto-Optical Deflection and Modulation of Coherent Light" appearing on pages 325-335 of the same issue of Applied Optics. The stepped array AO deflector was also disclosed inn U.S. Pat. No. 3,493,759. Since the stepped phased array transducers are difficult to implement in practice, simpler fabrication techniques have been proposed. These are disclosed in U.S. Pat. No. 4,381,887, entitled, "Simplified Acousto-Optic Deflector using Electronic Delays," and U.S. Pat. No. 4,671,620, entitled, "Phased-Array Acousto-Optic Bragg Cell".
All of the prior art have limited the discussion on the use of phased array for increasing the bandwidth of AO deflectors by operating at the tangential phase matching; i.e., the selection of the spacing of the phased array element according to Eq. (1).
It is possible to achieve tangential phase matching by combining phased array transducers and birefringent diffraction. The net effect is to shift the acoustic frequency for tangential phase matching. The phased array birefringent deflector is described in the following articles: "Birefringent Phased Array Bragg Cells," 1985 IEEE Ultrasonics Symposium Proceedings, pages 381-384 and "Generalized Phased Array Bragg Interaction in Anisotropic Crystals," 1991 Proceedings of SPIE, Vol. 1476, pages 178-179.
Recent development of AO devices has been focused on integrated optic or guided wave structure, i.e., the interaction between surface acoustic waves (SAW) and guided optical waves. The integrated optic AOTF is described in an article by Y. Ohmachi and J. Noda, entitled, "LiNbO.sub.3 TE-TM Mode Converter Using Collinear Acousto-Optic Interaction" appearing in IEEE Journal of Quantum ElecLronics, Vol. QE-13, pp. 43-46, 1977. To date, all integrated AOTF work has been restricted to LiNbO.sub.3 using collinear birefringent AO interaction. The use of phased array transducer for increasing the integrated AO deflector bandwidth has been discussed, for example, in an article entitled, "Efficient Wideband Guided-Wave Acousto-Optic Bragg Diffraction Using Phased-Surface Array in LiNbO.sub.3 Waveguide," Appl. Opt., Vol. 16, pp. 1297-1304, May, 1977.